Overland Kunming to Hong Kong
South West China is, in some ways, the most exotic area of China, largely populated by various ethnic minorities with interesting handicrafts, languages, folk music and so on. It is also a poor area, relatively cheap to travel in overall.
Some areas have been or are being developed for tourism; there you can expect to find some modern amenities, Western food, English speakers, and (by Chinese standards) high prices. Once you get out of those areas, expect none of the above.
The two end points, Kunming and Hong Kong, both have good air links to various places. See their articles and Discount airlines in Asia for details. Both also have rail connections to the rest of China.
Kunming to Nanning
From Kunming, you can take a train through sensational mountain scenery, to the provincial capital Nanning, the capital of Guangxi. From Nanning, it is a short trip by bus or train to tourist center Guilin.
There are also flights possible to both Nanning and Guilin from most major cities in China.
To Guilin via Guizhou
From Kunming, you can travel either by train or bus into Guizhou, a relatively poor and untouristed province with many ethnic minorities, then by bus through less travelled country to Guilin. Places en route include:
The overnight Kunming-Anshun bus is not recommended; it is at least 16 hours and neither clean nor comfortable. Take the train or break up the journey with an intermediate stop.
Guilin to Hong Kong
From Guilin, continue to the tourist town Yangshuo, then:
Either way, you can reach Hong Kong.
Alternate route, Nanning to Hong Kong along the coast
Kunming to Hong Kong by (mostly) air
The fastest reasonably priced way from Kunming to Hong Kong or vice versa is by a plane/ferry combination, about 4 hours / 850+ RMB. You could fly direct to Hong Kong, but it is usually significantly cheaper to land in Shenzhen and take a ferry.
Many foreigners in China need to get a new visa every 6 or 12 months and Hong Kong or Macau are good places for this. For most passports you can get a multiple entry business visa with just a photo and money. The plane/ferry combination is the usual method for such "visa runs".
Kunming - Hong Kong
That's it. For about 850 RMB you can go to Hong Kong.
A very convenient way to book the ferry and plane ticket together is at the China Travel Service (HK). It is easy to find as it is placed directly in the opposite of the Chunking Mansions in Hong Kong Kowloon where many backpackers stay. Turbojet Ferry Schedule
These areas are generally quite poor and petty theft is common. Take precautions against pickpockets and watch for things like razor attacks on luggage during bus or train rides.
Armed robbery is rare, but has been reported. Victims are generally lone travellers, often female. Try to stay in a group, especially on treks through isolated areas.
Altitude sickness is a risk in some areas. especially if you go west of Kunming up toward the Himalayas.
Both cities are also well connected to the Chinese rail network. From Kunming, the possibilities are: